The production of alkylene oxides via catalytic epoxidation of olefins in the presence of oxygen using silver based catalysts is known. Conventional silver-based catalysts used in such processes typically provide a relatively lower efficiency or “selectivity” (i.e., a lower percentage of the reacted alkylene is converted to the desired alkylene oxide). In certain exemplary processes, when using conventional catalysts in the epoxidation of ethylene, the theoretically maximal efficiency towards ethylene oxide, expressed as a fraction of the ethylene converted, does not reach values above the 6/7 or 85.7 percent limit. Therefore, this limit had long been considered to be the theoretically maximal efficiency of this reaction, based on the stoichiometry of the following reaction equation:7C2H4+6O2→6C2H4O+2CO2+2H2O
cf. Kirk-Othmer's Encyclopedia of Chemical Technology, 4th ed., Vol. No. 9, 1994, p. 926.
Alkylene oxide catalysts comprise three main chemical components: silver, a carrier, and solid promoter packages. Certain “high efficiency” or “high selectivity” modem silver-based catalysts are highly selective towards alkylene oxide production. For example, when using certain modern catalysts in the epoxidation of ethylene, the theoretically maximal efficiency towards ethylene oxide can reach values above the 6/7 or 85.7 percent limit referred to, for example 88 percent or 89 percent, or above. As used herein, the terms “high efficiency catalyst” and “high selectivity catalyst” refer to a catalyst that is capable of producing an alkylene oxide from the corresponding alkylene and oxygen at an efficiency greater than 85.7 percent. The observed actual efficiency of a high efficiency catalyst may fall below 85.7 percent under certain conditions based on process variables, catalyst age, etc. However, if the catalyst is capable of achieving at least an 85.7 percent efficiency, it is considered to be a high efficiency catalyst. Such highly efficient catalysts, which may comprise as their active components silver, rhenium, at least one further metal, and optionally, a rhenium co-promoter, are disclosed in EP0352850B1 and in several subsequent patent publications. “Promoters,” sometimes referred to as “inhibitors” or “moderators,” refer to materials that enhance the performance of the catalysts by either increasing the rate towards the desired formation of alkylene oxide and/or suppressing the undesirable oxidation of olefin or alkylene oxide to carbon dioxide and water, relative to the desired formation of alkylene oxide. As used herein, the term “co-promoter” refers to a material that—when combined with a promoter—increases the promoting effect of the promoter. In addition, promoters may also be referred to as “dopants.” In the case of those promoters that provide high efficiencies, the terms “high efficiency dopants” or “high selectivity dopants” may be used.
“Promoters” can be materials that are introduced to catalysts during the preparation of the catalysts (solid phase promoters, also referred to as “catalyst promoters” herein). In addition, “promoters” can also be gaseous materials that are introduced to the epoxidation reactor feed (gas phase promoters). In one example, an organic halide gas phase promoter may be added continuously to the epoxidation reactor feed to increase the catalyst efficiency. For silver-based ethylene epoxidation catalysts, both solid and gas phase promoters are typically required in any commercial processes.
Conventional catalysts have relatively flat efficiency curves with respect to the gas phase promoter concentration in the feed, i.e., the efficiency is almost invariant (i.e., the change in efficiency with respect to a change in gas phase promoter concentration in the feed is less than about 0.1%/ppm) over a wide range of promoter concentrations, and this invariance is substantially unaltered as reaction temperature is changed (i.e., the change in efficiency with respect to a change in reaction temperature is less than about 0.1%/° C.) during prolonged operation of the catalyst. However, conventional catalysts have nearly linear activity decline curves with respect to the gas phase promoter concentration in the feed, i.e., with increasing gas phase promoter concentration in the feed, temperature has to be increased or the alkylene oxide production rate will be reduced. Therefore, when using a conventional catalyst, for optimum efficiency, the gas phase promoter concentration in the feed can be chosen at a level at which the maximum efficiency can be maintained at relatively lower operating temperatures. Typically, the gas phase promoter concentration can remain substantially the same during the entire lifetime of a conventional catalyst. Alternatively, the reaction temperature may be adjusted to obtain a desired production rate without any substantial impact on efficiency.
By contrast, high efficiency catalysts tend to exhibit relatively steep efficiency curves as a function of gas phase promoter concentration as the concentration moves away from the value that provides the highest efficiency (i.e., the change in efficiency with respect to a change in gas phase promoter concentration is at least about 0.2%/ppm when operating away from the efficiency maximizing concentration). Thus, small changes in the promoter concentration can result in significant efficiency changes, and the efficiency exhibits a pronounced maximum, i.e., an optimum, at certain concentrations (or feed rates) of the gas phase promoter for a given reaction temperature and catalyst age as well as other conditions such as feed gas composition. Moreover, the efficiency curves and the optimum gas phase promoter concentration tend to be strong functions of reaction temperature and are thus significantly affected if reaction temperature is varied, for example, to compensate for decreases in catalyst activity, (i.e., the change in efficiency with respect to a change in reaction temperature can be at least about 0.1%/° C. when operating away from the efficiency maximizing promoter concentrations for the selected temperatures). In addition, high efficiency catalysts have exhibited significant activity increases with increases in the gas phase promoter concentration in the feed, i.e., with increasing gas phase promoter concentration in the feed, temperature has to be decreased or the production rate will increase.
Variations in high-efficiency catalyst properties can yield undesirable variations in catalyst activities, which in turn, can result in different catalyst operating temperatures. Variations in catalyst operating temperatures can result in the need to use different gas-phase promoter concentrations and different optimum values for such concentrations. Optimum process conditions developed for catalysts having particular physical or chemical property values may be sub-optimum for catalysts having different values for such properties.
It is typical for high-efficiency alkylene oxide catalysts to exhibit variability of physical properties, such as carrier specific surface area, silver specific area, carrier porosity, carrier total pore volume, and shape even when all manufacturing process parameters are well-controlled. This variability is attributable, at least in part, to natural variations in the raw materials used to prepare the carrier and the inherent variability of the carrier manufacture process. If such variations are not handled properly, it may result in variable and less predictable alkylene oxide catalyst properties and performance.
The physical properties of high-efficiency alkylene oxide catalysts, such as the carrier specific surface area (e.g., surface area/carrier mass) may vary from batch to batch of catalyst, resulting in variable and less predictable process performance. The inconsistency of catalyst performance may result in difficulty in running reaction systems comprising multiple catalyst batches. It particular, such inconsistencies can result in less stable operation, lower selectivity, and shortened catalyst life cycle. It is typical for a multi-reactor system to share a common feed gas mixture. When the catalyst activity varies significantly among different reactors and/or among the tubes within the same reactor, it could lead to hot spots or runaway reactions in catalyst regions of relatively high catalyst activity. For high efficiency alkylene oxide catalysts in particular, catalyst activity and efficiency both heavily depend on the level of the gas phase promoters. The optimum levels of such gas-phase promoters are in turn strongly dependent on reaction temperature. Thus, a higher variation in catalyst intrinsic activity may lead to problems with reactor temperature control and/or continued sub-optimized operation. The consequences could be loss of both short-term and long-term catalyst performance.
Certain references have disclosed multiple catalyst formulations in which the catalyst concentration of a particular promoter varies with the carrier specific surface area. Other references have characterized promoter concentrations based on their ratios to carrier specific surface area. However, such references have not acknowledged the advantages of or suggested adjusting promoter concentrations based on catalyst properties—such as catalyst physical properties like carrier specific surface area and silver specific surface area—when formulating a high-efficiency alkylene oxide catalyst to achieve consistent predictable performance. Nor do such references teach how to make different catalyst batches having substantially uniform performance properties when the carrier properties such as carrier specific surface areas are significantly different. Nor have such references distinguished between promoters whose concentrations can beneficially be scaled based on catalyst carrier specific surface area and those that cannot. In addition, they have failed to acknowledge or appreciate the advantages of scaling the concentrations of particular promoters, such as cesium, rhenium, sodium, lithium and/or sulfur or sulfate, with respect to any catalyst physical properties, including carrier specific surface area. Thus, a need has arisen for a catalyst preparation method and reaction system that achieves catalyst performance expectations, maintains predictable low risk operating characteristics with respect to temperature, gas phase promoter levels, and aging, and/or which provides the ability to beneficially utilize catalyst carrier lots with varying carrier specifications (e.g., carrier specific surface area). In addition, it is desirable to accelerate catalyst manufacturing timelines by using prior catalyst data in situations where extensive lab-scale testing for the purpose of optimizing solid-phase promoter levels is not available prior to catalyst production.